1. Field
The present disclosure relates to fuel cell systems and methods of operating the same, and more particularly, to fuel cell systems in which a low open circuit voltage (OCV) is maintained in a start-up mode and in a shut-down mode, and methods of operating the same.
2. Description of the Related Art
Fuel cells are a type of power generation device that produces electricity continuously, if a fuel is supplied thereto, and that directly convert chemical energy of the fuel into electrical energy, according to a chemical reaction. However, the voltage produced in one such fuel cell is generally not sufficient for practical use. Thus, a plurality of fuel cell units (unit cells) are connected in series, to form a stack. Each of the unit cells includes an anode, a cathode, an electrolyte membrane disposed therebetween, and a plates having channels to supply an oxidant and a hydrogen-containing fuel to the anode and the cathode, respectively. In the unit cells, a reverse reaction of water electrolysis is performed, via the electrolyte membrane, so that electricity is produced.
A direct current (DC) generated by a fuel cell can be supplied to a power conditioning system (PCS), so as to generate an alternating current (AC), which can then be supplied to a load. The PCS can include a DC/DC converter and/or a DC/AC inverter. The DC/DC converter boosts a voltage of the DC generated by the fuel cell, so as to be an appropriate voltage for a given load. The DC/AC inverter converts the voltage-boosted DC into AC.
Generally, when the fuel cell system operates under a rating load, a voltage generated by the unit cells is increased. Thus, a voltage input to the PCS is varied, and operating conditions of the PCS are accordingly varied. In this way, if the voltage input to the PCS is increased, and a gain of a closed circuit is increased, so that a phase margin is reduced and a stabilization of the PCS is lowered.
In particular, the PCS operates in the most unstable condition, at a high open circuit voltage (hereinafter referred to as an OCV) that is generated before a load is applied to the PCS. Also, since the OCV of a fuel cell stack, which is the sum of OCVs generated in respective unit cells, is a criterion for determining a maximum voltage of the PCS, if the OCV of the fuel cell stack is increased, operating voltages of the components of the PCS are increased. As such, the entire configuration of the PCS and an algorithm for controlling the PCS are more complicated.